APphoto_APTOPIX Afghanistan Election

Afghans erect an election poster of presidential election candidate Abdullah Abdullah in Kabul on Sunday as campaigning officially opened. (Massoud Hossaini / Associated Press / February 2, 2014)

KABUL, Afghanistan — The campaign to succeed Afghan President Hamid Karzai officially began Sunday amid deep uncertainty about the prospects for a peaceful election after two aides to a leading candidate were killed in the normally peaceful western city of Herat.

The aides to Abdullah Abdullah, a prominent opposition politician who was runner-up to Karzai in the disputed 2009 election, were shot and killed by unknown gunmen outside a campaign office where they had just concluded a meeting Saturday evening, officials in Herat said.

The killings cast an immediate shadow over the campaign for the April 5 election, a crucial test of the Afghan government’s ability to manage a stable transfer of political power as U.S.-led NATO troops prepare to withdraw from Afghanistan by the end of the year.

Eleven candidates are running to replace Karzai, who has led Afghanistan for 13 years but has clashed bitterly with his American backers in recent months. Karzai, who is barred by law from seeking another term, has accused the U.S. government of trying to undermine Afghan institutions and delayed signing a bilateral security pact that could see some U.S. forces remain in Afghanistan past 2014 to conduct military training and counterterrorism operations.

As campaign posters began popping up across Kabul, Abdullah was widely seen as one of the front-runners, given the 31% of the vote he received in 2009 and his strong support among ethnic Tajiks in northern Afghanistan. Another leading candidate is Ashraf Ghani, a former finance minister who leads an influential committee charged with overseeing the security transition from NATO to Afghan forces.

Although no group immediately claimed responsibility for Saturday’s slayings, Taliban insurgents have vowed to derail the election.

Sayed Fazel Sancharaki, a spokesman for Abdullah’s political organization, the National Coalition of Afghanistan, said the victims — Faiz Ahmad Hamdard, a campaign manager, and his 19-year-old driver — had been threatened “many, many times” by text message and phone calls over the past week.

“We see it as a great loss,” Sancharaki said.

Hamdard was a senior aide to Abdullah in Herat, heading 12 campaign offices in the western province, Sancharaki said. His driver, Shujahee, who like many Afghans uses just one name, was a nephew of legendary Afghan warlord Ahmad Shah Massoud, who helped lead the war against Soviet occupation in the 1980s, fighting alongside Abdullah.

The ranking United Nations official in Afghanistan, Jan Kubis, condemned the killings and called on Afghan security forces to exercise heightened vigilance over the coming weeks.

“This [cowardly] action constitutes a violent intimidation of electoral candidates and their supporters, and cannot be tolerated,” Kubis said.

Other candidates vying to succeed Karzai include his elder brother, Abdul Qayum Karzai, a businessman who ran the family’s restaurants in the United States but hasn’t received the president’s backing; former foreign minister Zalmai Rassoul, a well-traveled diplomat seen as close to Karzai; and Abdul Rab Rasoul Sayyaf, a conservative cleric who has stirred controversy because of allegations of links to radical Islamists including Osama bin Laden.

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Baktash is a Times special correspondent. Times staff writer Shashank Bengali contributed to this report from New Delhi.